1. Field of the Invention
The present invention is generally concerned with producing an optical quality plastics material element in which the refractive index varies continuously in at least one direction.
It is more particularly, but not necessarily exclusively directed to the case where the synthetic material element is adapted to be used to produce an ophthalmic lens.
2. Description of the Prior Art
It is known that varying the refractive index within an ophthalmic lens makes it possible, for the same degree of correction, to reduce the maximum thickness and therefore the weight, for example, to the benefit of user comfort.
It is also known that at present only polymerization techniques can obtain an index gradient over a distance sufficient to meet the requirements of ophthalmic optics.
At present, however, these techniques do not allow sufficient control over the index gradient.
In particular, they do not allow a specific index gradient profile to be obtained that is variable at will but reliably reproducible.
Japanese patent application No 60-200.838, for example, describes the use as the base material of a mixture of two monomers of sufficiently different molecular weight and yielding homopolymers with different refractive indexes; a certain radial distribution of mass is achieved in these monomers by centrifuging and then fixed by polymerization.
Apart from the fact that a process of this kind is relatively difficult to use it does not enable a sufficiently accurate specific index gradient profile to be obtained and can only produce an index gradient in a radial direction relative to the axis of the synthetic material element obtained; in practise it is at best suitable only for making relatively small synthetic material elements.
Japanese patent application No 60-162.610 also proposes the use as the starting material of a mixture of at least two monomers yielding homopolymers with different refractive indexes, the latter being chosen to have different reaction speeds and the mixture being subjected to a gravitational field able to modify their distribution as polymerization proceeds.
This process is also difficult to control accurately.
Finally, Japanese patent application No 60-175.009 proposes the use of diffusion.
To be more precise Japanese patent application No 60-175.009 proposes to diffuse into a base material formed of at least one partially polymerized monomer able to yield a homopolymer with refractive index N1 in the gel state an additive material itself formed of at least one monomer able to yield a homopolymer with refractive index N2 different from N1 and then to finish the base material treated in this way.
With a process of this kind it appears difficult, if not impossible, in practise to operate on the parameters such as the temperature and the state of the gel likely to influence the polymerization and therefore to impose on the diffusion profile the variations needed to obtain the required refractive index profile of the synthetic material element finally obtained.
A general object of the present invention is an arrangement using a diffusion process to obtain accurately, reliably and reproducibly a given index gradient profile.